Article pubs.acs.org/jced
Cite This: J. Chem. Eng. Data 2019, 64, 2742−2749
Heat Capacity and Phase Behavior of Selected Oligo(ethylene glycol)s Vać lav Pokorný,† Paulo B. P. Serra,† Michal Fulem,† Carlos F. R. A. C. Lima,‡,§ Luis M. N. B. F. Santos,‡ and Kveť oslav Růzǐ cǩ a*,†
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†
Department of Physical Chemistry, University of Chemistry and Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic ‡ CIQUP, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal § Department of Chemistry & QOPNA, University of Aveiro, Aveiro 3810-193, Portugal S Supporting Information *
ABSTRACT: This work aims to provide reliable heat capacities for ethylene glycol and selected oligo(ethylene glycol)s (diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, and hexaethylene glycol), which are industrially important chemicals produced on a large scale. Besides, new data extend the database needed for a better understanding of complex behavior of compounds capable of forming hydrogen bonds. Isobaric heat capacities of ethylene glycols were measured with a Tian−Calvet-type calorimeter in the temperature range of 260 to 358 K. The phase behavior was investigated with a heat-flux differential scanning calorimeter. A simple additive estimation method for liquid heat capacity of oligo(ethylene glycol)s was developed and tested through comparison with newly measured liquid heat capacities of polyethylene glycols 400 and 600.
glycol, Gallaugher and Hibbert10 suggested existence of intramolecular bonds as early as in 1937 and significant role of intramolecular bonding in neat ethylene glycols is advocated also for example by Kozlowska et al.11 using Car−Parrinello molecular dynamics simulations. On the other hand, the study by Hubbard et al.12 (though not focused on ethylene glycols) favored intramolecular bonds over intermolecular ones for molecules with maximum five rotatable bonds between the donor and acceptor; for larger distances, intramolecular bonds were found negligible. In the case of oligo(ethylene glycol)s, intramolecular hydrogen bonding competes with hydrogen bonding between the terminal donors and −O− acceptors, which leads to parallel aligning of molecules enhanced by OH···O hydrogen bonds for tetra- and higher ethylene glycols (based on analysis of permittivity spectra13). The abovementioned papers are merely examples of experimental and computational effort devoted to oligo(ethylene glycol)s and it is outside the scope of this paper to provide thorough analysis of this problematics. Instead, this paper aims in providing reliable experimental heat capacities of oligo(ethylene glycol)s in the liquid state in the technologically relevant temperature range. Apart from ethylene glycol, heat capacities of oligo(ethylene glycol)s can be correlated as a function of number of
1. INTRODUCTION Ethylene glycol and oligo(ethylene glycol)s of general formula (H−[O−CH2−CH2]n−OH) with n < 7 are industrially important compounds used as solvents, low-volatile working fluids in cooling systems, natural gas drying agents, in polyester production and so forth. 1−3 The heat capacities are indispensable for the evaluation of the variation of thermodynamic properties with temperature. An extensive collection of critically assessed heat capacity data was published,4−6 yet the uncertainty of available literature values is significant in the case of oligo(ethylene glycol)s. Besides their technological importance, oligo(ethylene glycol)s are of great interest because of the existence of two −OH groups and varying number of −O− groups, promoting the establishment of multiple hydrogen bonds. Ethylene glycol, included in the study for comparison, was studied by a number of researchers; however, the structure and population of resulting ethylene glycol oligomers kept together by hydrogen bonding is not unambiguously known. Based on near-infrared spectroscopy, Chen et al.7 concluded that no intramolecular bonding is present in the liquid phase, it is however clear that ethylene glycol molecules are significantly engaged in a hydrogen bond network; the fraction of unbounded OH groups was estimated to be merely 3.2% by Kaiser et al.8 using molecular dynamics simulations with OPLS-AA-SEI-M force field, and Kollipost et al.9 recently reported symmetric dimers held by four hydrogen bonds as a most stable oligomer. In the case of diethylene © 2019 American Chemical Society
Received: February 8, 2019 Accepted: April 25, 2019 Published: May 3, 2019 2742
DOI: 10.1021/acs.jced.9b00140 J. Chem. Eng. Data 2019, 64, 2742−2749
Journal of Chemical & Engineering Data
Article
Table 1. Sample Descriptions compound
molar mass
CAS number
supplier
mole fraction puritya
mole fraction purityb
water contentc,d
ethylene glycol diethylene glycol triethylene glycol tetraethylene glycol pentaethylene glycol hexaethylene glycol polyethylene glycol 400 polyethylene glycol 600
62.07 106.12 150.17 194.23 238.28 282.33 notef noteh
107-21-1 111-46-6 112-27-6 112-60-7 4792-15-8 2615-15-8 25322-68-3 25322-68-3
Aldrich Aldrich Aldrich Aldrich Aldrich TCI Aldrich Aldrich
0.9981 0.9998 0.999 0.992 0.993 0.9850 n.a.g n.a.g
0.9989 1.0000 0.9994 0.9986 0.9930 0.9952 n.a.g n.a.g
5.35 × 10−5 e
1.11 × 10−4 2.25 × 10−4 4.90 × 10−5 e
1.51 × 10−4 1.75 × 10−4
a
Purity stated by the supplier. bGas−liquid chromatography using Hewlett-Packard 6890 gas chromatograph equipped with column HP5 crosslinked 5% PHME siloxane, length 30 m, film thickness 0.25 μm, i.d. 0.32 mm, and FID detector. cDried using molecular sieves. dKarl-Fischer analysis by Metrohm 831. eWater content was below the detection limit of Karl-Fischer analysis by Metrohm 831 because of the limited amount of the sample available. fNumber average Mn 340 g·mol−1, and mass average of molar mass Mw = 378 g·mol−1 (both results based on gel permeation chromatography performed with an Agilent 1260 Infinity GPC/SEC system equipped with PLGel 100 Å/5 μm column (Varian)). gPurity determination not applicable. hNumber average Mn = 555 g·mol−1 and mass average of molar mass Mw = 595 g·mol−1 (both results based on gel permeation chromatography described in note f).
[O−CH2−CH2] groups and used for estimation of heat capacities of higher oligo(ethylene glycol)s, as evidenced by the case of polyethylene glycols 400 and 600. The heat capacity studies were complemented with the analysis of phase behavior. The heat capacities of hexaethylene glycol and enthalpies of fusion for all oligo(ethylene glycol)s are reported for the first time.
2.3. Phase Behavior. The phase behavior of the selected glycols was investigated in the temperature range from 183 to 300 K using a heat flux differential scanning calorimeter TA Q1000 (TA Instruments, USA). The measurements were carried out using a continuous method14 with a heating rate of 2 K·min−1. The temperature and enthalpy calibrations of the device were performed using water, gallium, naphthalene, indium, and tin. Based on calibration results, the uncertainty in the determination of phase transition temperatures T and enthalpies ΔH are estimated to be u(T) = 0.3 K (0.68 level of confidence) and U c (ΔH) = 0.03·ΔH (0.95 level of confidence), respectively. The samples of about 10 mg were enclosed in hermetic aluminum pans. The phase transition temperatures were determined as the onset of the corresponding peaks.
2. EXPERIMENTAL SECTION 2.1. Description and Characterization of the Samples. The ethylene glycols studied were of commercial origin and were used as received except for drying over 0.4 nm molecular sieves because their purity was found satisfactory for heat capacity evaluation (see Table 1). Sample loading into calorimetric vessels and pans was performed in a glovebox (MBraun LabStar) under a dry nitrogen atmosphere. 2.2. Heat Capacity Measurements. A highly sensitive Tian−Calvet calorimeter (SETARAM μDSC IIIa) was used for the measurement of heat capacities using a continuous method.14 This method should yield identical results as a more time-consuming incremental (temperature step) method, assuming that the calorimeter baseline changes linearly with temperature. This was confirmed for the SETARAM μDSC IIIa calorimeter and the temperature range from 260 to 358 K used in this study.15 A typical mass of the sample used was 500 mg. A vessel with the sample and an empty reference container were used in each measurement using a scanning temperature rate of 0.35 K·min−1. The same blank correction was considered for the sample and calibration experiments using synthetic sapphire SRM 720 (performed using the same experimental conditions). Performance of the calorimeter is regularly checked by measurement of compounds with wellestablished values (naphthalene,16 benzophenone,17 benzoic acid18). Based on these checking experiments, the combined expanded uncertainty of the heat capacity measurements is estimated to be Uc(Cpm) = 0.01·Cpm. The temperature dependence of the experimental heat capacity data was fitted to a polynomial equation Cpm R
i T y ∑ Ai+ 1jjj zzz k 100 { i=0 n
=
3. RESULTS AND DISCUSSION 3.1. Heat Capacities. Experimental heat capacities obtained in this work with SETARAM μDSC IIIa are listed in the Supporting Information in Table S1 and summarized along with literature values in Table 2. Figure 1 shows a good agreement (≤0.30%) for ethylene glycol in the overlapping temperature range between the literature data of Góralski and Tkaczyk26 and Takeda et al.23 and the experimental heat capacities of this work, thus confirming reliable performance of our experimental setup. Consequently, these three data sets were used to establish parameters of eq 1. Li et al.28 reported heat capacities for diethylene glycol, triethylene glycol, and tetraethylene glycol which are in good agreement with data of this work (deviations < 1%) and were therefore included in the correlation. Remaining sets were not included in the correlation for the following reasons. Data reported by Stephens and Tamplin21 deviate significantly from recommended data for ethylene glycol and for diethylene- to pentaethylene glycols. Similarly, values reported by Zaripov22 for diethylene and hexaethylene glycol deviate significantly from data of this work, therefore, heat capacities by Zaripov22 were not included in the final fit. Steele et al. reported heat capacities for diethylene glycol27 and triethylene glycol29 using power-compensated differential scanning calorimetry (DSC) over the temperature range extending from room temperature near to the critical point. In the case of triethylene glycol, only parameters of the polynomial equation were given,29 therefore, uncertainty of
i
(1)
where R is the molar gas constant (R = 8.3144598 J·K−1· mol−1) and Ai+1 the fitted coefficients. A good fitting of the data was found for a second-order polynomial (n = 2). 19
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DOI: 10.1021/acs.jced.9b00140 J. Chem. Eng. Data 2019, 64, 2742−2749
Journal of Chemical & Engineering Data
Article
Table 2. Overview of the Experimental Heat Capacities of the Selected Oligo(ethylene glycol)s in the Temperature Range of the Present Study referencea
Nb
(Tmin−Tmax)/K
ur(Cpm)/%c
mole fraction purityd
method
0.9970 0.9990 Nosp Nosp Nosp 0.9980 0.9980 0.9989
isoperibol DSC Tian−Calvet adiabatic adiabatic DSC Tian−Calvet Tian−Calvet
0.999 0.9990 Nosp 0.99 1.0000
DSC DSC Tian−Calvet DSC Tian−Calvet
Nosp Nosp 0.9999 0.9990 0.9994
DSC Tian−Calvet DSC DSC Tian−Calvet
0.9990 Nosp 0.9990 0.9986
DSC Tian−Calvet DSC Tian−Calvet
0.9910 0.9930
DSC Tian−Calvet
Nosp 0.9952
Tian−Calvet Tian−Calvet
n.a.h
Tian−Calvet
e
Parks and Kelley20 Stephens and Tamplin21 Zaripov22 Takeda et al.23 Nan et al.24 Yang et al.25 Góralski and Tkaczyk26 this work
6 15 3 16 21 9 41 40
Steele et al.27 Stephens and Tamplin21 Zaripov22 Li et al.28 this work
14 5 3 6 42
Stephens and Tamplin21 Zaripov22 Steele et al.29 Li et al.28 this work
5 3 Eq.g 6 38
Stephens and Tamplin21 Zaripov22 Li et al.28 this work
5 3 6 42
Stephens and Tamplin21 this work
13 42
Zaripov22 this work
3 29
this work
30
this work
22
Ethylene Glycol 262.00−293.00
